The present invention relates to an ethylene-propylene rubber composition obtained by vulcanization with sulfur, in which the rubber component totally or mainly composed of an ethylene-.alpha.-olefin-diene copolymer (hereinafter "EPDM" for short) is incorporated with a zinc oxide, a carbon black and other ingredients. In particular, this invention is concerned with an ethylene-propylene rubber composition best suited for high-pressure reinforcing hoses (ex. a brake hose as shown in FIG. 1), gaskets, and so forth. The vulcanized rubber layers of the hoses are required to have improved compression set resistance because the hoses have squeezed hose-joints. The gaskets are required to maintain good sealing properties over a prolonged period.
This invention will be explained specifically, but not exclusively, with reference to a brake hose.
In the present disclosure, unless otherwise stated, the unit "parts" is given by weight, and the unit "PHR" is a quantity with respect to 100 parts of rubber polymer.
The inner rubber layers of brake hoses have been generally made of styrene-butadiene rubber (hereinafter "SBR" for short). But today the replacement of SBR by EPDM is being under investigation so as to satisfy demands of improving heat-resistance and other properties of the hoses.
However, applying general EPDM compositions to the inner layer of a brake hose causes a problem that maintaining good sealing properties between an inner rubber layer 1 of a brake hose and a nipple 11a of a hose joint 11 under such severe conditions as stated above is difficult, especially preventing compression set of the rubber layer is difficult.
We have already reached an invention involving a inner layer made of a specific type of EPDM rubber vulcanized with peroxides in order to solve the above problem and filed the invention as a Japanese patent application (see Japanese published unexamined patent application sho 63(1988)-241051).
However, the peroxides used for vulcanization (of which safety care should be taken) are not only intractable but must be handled in an inert gas atmosphere as well, resulting in considerable productivity drops.
This problem may be solved by using sulfur vulcanization, but those skilled in the art are asserting that even with this it is impossible to impart sufficient compression-set resistance to EPDM rubber.